One type of surge protection devices are typically used on sensitive power mains. These surge protection devices must divert most of the power of a transient event, also referred to as an overvoltage event. These transient events include, for example, lightning strikes. Transient events typically take place in the time range of <1 ms. Normatively, this type of transient event is modeled, for example, as a reference surge current with a 10/350 μs wave shape profile in the kA range.
For use as a surge protection device, as a rule varistors and/or spark gaps are used. However, with these varistors or spark gaps a grid follow current also occurs, which is caused by a low impedance discharge path. However, a surge protection device is also used to prevent or limit and switch off this grid follow current without a backup fuse of the surge protection device or the installation to be protected being triggered.
Conventional spark gaps have a low arc burning voltage and are not therefore able to control grid follow currents. These conventional spark gaps are extinguished in the alternating voltage passing through zero (mains voltage) and trigger a backup fuse, however, and in particular in the case of use on powerful electric mains. Nevertheless, this spark gap technology has the advantage that the components are very powerful. Thus commercially available products make it possible to divert surge currents up to 50 kA.
An improvement in the ability to extinguish grid follow current is provided by spark gaps with arc splitter chambers on the basis of arc multiplication. However, these have the problem that hot ionized gases are blown out in an explosive manner.
Another improvement of the ability to extinguish grid follow current is provided by encapsulated spark gaps in pressure-resistant housings. With these encapsulated spark gaps the arc voltage is raised into the range of the mains voltage by a pressure buildup in the spark gap and a grid follow current development is thus suppressed.
However, a grid follow current can develop at the end of a transient event, when the surge current is no longer sufficient to maintain the pressure and thus the arc voltage. That is, the grid follow current cannot be reliably switched off. In addition, encapsulated spark gaps have large electrode spacings, with which a poorer reaction behavior is associated compared to “simple” spark gaps.
In another technical field, namely, telecommunications, positive temperature coefficient devices (“PTCs”) are used in combination with gas discharge tubes in overvoltage protection. However, this is a field of application of overvoltage protection in which no high electric power is available on the part of the mains. In overvoltage protection PTCs are used for the current limiting of long-lasting faults, that is faults that last much longer than transient events, i.e., >>1 ms. The PTCs used there have ohmic resistances of more than 1 ohm and are able to carry rated currents in the range of a few amperes. Due to the low powers, these ohmic resistances of the PTCs used do not represent a major problem at switchgear cabinet temperature.
The manufacturers in this field of technology are thereby endeavoring to render possible current densities that are as high as possible with low volumes, in order to reach low W/R values or i2t values (specific energy or joule integral) up to the switching threshold of the PTC.
Accordingly, an object of the presently disclosed embodiments is to provide an improved surge protection device which has the advantages of a spark gap arrangement for the surge protection and an improved grid follow current extinction behavior. Another object is furthermore to avoid the unnecessary triggering of an existing backup fuse of the arrangement.